1. Field of the Invention
The present invention relates to a brake system for use in a vehicle including one front axle and two rear axles and, in particular, to a wheel lock prevention system which prevents the locking of the wheels.
2. Discussion of the Related Art
FIGS. 1 and 2, respectively, show first and second conventional anti-lock brake systems for use in a large-sized vehicle including three axles.
In the first conventional system shown in FIG. 1, the front side axle thereof includes right and left front wheels 151, 152, the first rear axle thereof includes right and left driving wheels 153, 154, and the second rear axle thereof includes right and left follower wheels 155, 156. Also, the first conventional system includes first and second speed sensors S101, S102 which are respectively used to detect the speeds of rotation of the right and left front wheels 151, 152 and, at the same time, includes third and fourth speed sensors S103, S104 respectively used to detect the rotational speeds of the right and left driving wheels 153, 154. The values of the rotational speeds detected by the speed sensors S101-S104 are output as speed signals to a control part (not shown).
As shown further in FIG. 1, a first brake control device part 110 is interposed through a relay valve 108 between a brake valve 107 and the respective wheel cylinders of the right and left front wheels 151, 152. The first brake control device part 110 includes a first air master cylinder 111, which converts air pressure from a brake valve side into the liquid pressure and supplies it to a wheel cylinder side, and a first modulator 112, which controls the liquid pressure to be fed to the first air master cylinder 111. The first modulator 112 can be operated properly in accordance with an operation signal output from the control part.
Similarly, between the brake valve 107 and the respective wheel cylinders of the right drive and follower wheels 153 and 155, through a relay valve 109, there is interposed a second brake control device part 120, which comprises a second air master cylinder 121 and a second modulator 122. The second brake control device part 120 is used to control brake pressures to be applied to the respective wheel cylinders of the right drive and follower wheels 153 and 155.
Further, between the brake valve 107 and the respective wheel cylinders of the left drive and follower wheels 154 and 156, through the relay valve 109, there is interposed a third brake control device part 130, which includes a third air master cylinder 131 and a third modulator 132. The third brake control device part 130 is used to control brake pressures to be applied to the respective wheel cylinders of the left drive and follower wheels 154 and 156.
In the first conventional system, on the front wheel side, the control part carries out a select-low control to operate an operation signal to be applied to the first brake control device part 110 in accordance with the speed signal from one of the right and left front wheels 151, 152 having a lower rotational speed. And, the right driving wheel 153 and right follower wheel 155 are controlled by the second brake control device part 120 independently of the left side wheels. Also, the left driving wheel 154 and left follower wheel 156 are controlled by the third brake control device part 130 independently of the right side wheels.
Next, description will be given below of the second conventional system shown in FIG. 2.
In FIG. 2, parts given the same designations as those in FIG. 1 are similar in structure and operation to those employed in the first conventional system and thus the description thereof is omitted.
In the second conventional system, the right and left follower wheels 155, 156 are not in communication with the second and third brake control device parts 120, 130, respectively, but they are in communication with the first brake control device part 110 on the front wheel side.
Now, in the first conventional system, since the drive and follower wheels on the rear side are controlled by the same pipe (a pipe D interposed between the brake valve 107 and relay valve 109), if the pipe D is defective, then the four wheels 153, 154, 155, 156 on the rear wheel side will fail, which may have serious consequences.
Also, when the first conventional system includes an acceleration skid control mechanism, unless cut valves 161 and 162, each having an electromagnetic valve, are newly interposed, respectively, between the right drive and follower wheels 153, 155 and left drive and follower wheels 154, 156, the same brake pressure applied to prevent skidding of the driving wheels 153, 154 is also supplied to the follower wheels 155, 156. This increases the number of parts and the costs of the system.
In the second conventional system, since the follower wheels 155, 156 are braked in accordance with the speed signals of the front wheels during the anti-lock control operation, the possibility of the follower wheels 155, 156 locking increases, and the braking performance and stability of the vehicle deteriorates.